I am hoping this is just a question about how a motor works and nothing is wrong. I recently purchased my first camper (3,800 lbs) and 2500HD, gas, Chevy truck. When towing in tow mode it appears the transmission stays in 5th gear and the motor operates at 2250 rpm. When going up a small hill, nothing really changes. When going up a medium hill and the engine begins to strain, the engine will abruptly make an additional noise and the gas mileage indicator goes from say 8 mpg to 5 mpg. However, it stays in 5th gear and the rpms stay about 2250. As the top of the hill is reached the noise gradually disappears. On steep hills, the noise will begin and a few seconds later the truck will downshift to 4th and rpms go to about 3300. My questions is, is this normal or should I be concerned with the additional noise that occurs without a change in gears? If it is normal, does anyone know what is causing the noise?

Unless you are really pulling steep grades with your set up no need to use tow haul mode. I pull my almost 9000 V-lite most of the time in regular mode till I encounter steeper hills. You will find the gas mileage is better also than if you leave in tow haul mode. The noise you maybe hearing may also be the air being drawer into the engine. Typically normal. Later RJD

Verret - I'm going to disagree that the torque converter is unlocking, if that was the cause of the noise increase the engine RPM's should change at the same time. I also disagree with not using "tow/haul", why did the manufacturer put it on the truck if there wasn't some benefit while towing? Now for the cause of the noise, IMO you are hearing the engine cooling fan which is thermostatically controlled. Next time it happens, closely watch your coolant temperature and I'll bet the temp goes up as you start to climb the hill and then comes back down shortly after the new noise starts.

I am hoping this is just a question about how a motor works and nothing is wrong. I recently purchased my first camper (3,800 lbs) and 2500HD, gas, Chevy truck. When towing in tow mode it appears the transmission stays in 5th gear and the motor operates at 2250 rpm. When going up a small hill, nothing really changes. When going up a medium hill and the engine begins to strain, the engine will abruptly make an additional noise and the gas mileage indicator goes from say 8 mpg to 5 mpg. However, it stays in 5th gear and the rpms stay about 2250. As the top of the hill is reached the noise gradually disappears. On steep hills, the noise will begin and a few seconds later the truck will downshift to 4th and rpms go to about 3300. My questions is, is this normal or should I be concerned with the additional noise that occurs without a change in gears? If it is normal, does anyone know what is causing the noise?

H

Tow / Haul mode just changes the shifts points in the tranny , stays in lower gears longer when starting out before shifting and down shifts at ealry points such as going up a hill or down . turn off your t/h mode and try and shift yourself and see if there's a difference in operation and noise . I up and down shift with the selector better then the t/h mode can do it .same same only better . also when your going up hill with the load expect mpg to drop. normal

Tow / Haul mode just changes the shifts points in the tranny , stays in lower gears longer when starting out before shifting and down shifts at ealry points such as going up a hill or down . turn off your t/h mode and try and shift yourself and see if there's a difference in operation and noise . I up and down shift with the selector better then the t/h mode can do it .same same only better . also when your going up hill with the load expect mpg to drop. normal

tow/haul also changes the operating pressure holding the clutches. Can you do that manually better than the PCM can?